If the Manor gets 50 KCAL from energy by eating the starfish how much energy does the clam get from eating the plankton

Energy that the clam get from eating the plankton is 5000 Kcal.

Explanation:

The right question should be:

If the Manta ray gets 50 KCal from energy by eating the starfish how much energy does the clam get from eating the plankton?

In an example of a marine ecosystem food chain, the energy flow may be

Planctom → Clam → Starfish → Manta ray

Taking into account that at each level of the chain only a tenth of the energy of the previous level is used, it has to be:

Planctom → Clam → Starfish → Manta ray

50.000 Kcal - 10% = 5000 Kcal - 10% = 500 Kcal - 10% =  500 Kcal

So the energy the clam got from the plankton was 5000 Kcal.